[PATCH v1] libstdc++: Implement std::layout_left_padded.
Tomasz Kaminski
tkaminsk@redhat.com
Thu Sep 11 07:40:00 GMT 2025
On Thu, Sep 11, 2025 at 9:03 AM Luc Grosheintz <luc.grosheintz@gmail.com>
wrote:
>
>
> On 9/9/25 9:48 AM, Tomasz Kaminski wrote:
> > On Mon, Sep 8, 2025 at 10:14 PM Luc Grosheintz <luc.grosheintz@gmail.com
> >
> > wrote:
> >
> >> I decided to deviate once (described in detail in the commit message).
> >> Without this deviation writing tests is very clumsy and sometimes not
> >> possible, e.g. when mixing uint8_t and dynamic padding values to check
> >> mandates w.r.t. representability. Previously, we discussed other ways of
> >> fixing the issue. The reason I chose this version is because I believe
> >> it's the smallest deviation from the standard; and we can deviate
> >> differently in followup commits.
> >>
> > Could you please submit the issue for this one? This is obviously wrong,
> > and I agree that you direction is least deviation from the standard.
> >
>
> It's been reported, I don't have a number yet.
>
I have seen that, thanks.
>
> >>
> >> ---------------------------------------------------------
> >>
> >> The tests contain a function:
> >>
> >> constexpr bool
> >> is_same_mapping(const auto& lhs, const auto& rhs)
> >> // ...
> >> for (size_t i = 0; i < lhs.extents().rank(); ++i)
> >> if (lhs.stride(i) != rhs.stride(i))
> >> return false;
> >> return true;
> >> }
> >>
> >> The above is accidentally quadratic, because computing all strides can
> >> be done in linear time, but not if one computes them one by one, then
> >> it's quadratic. It's annoyingly hard to do in linear time, because it
> >> requires both precise knowledge of the definition of `stride` for each
> >> layout; and ideally it also requires access to implementation details
> >> related to how we separate static and dynamic extents to be able to
> >> compute static products at compile time.
> >>
> > I think this is fine for tests, as long as they run in reasonable time,
> > which seems to be the case one.
>
> Fully agree. However, it seems like computing all strides
> in one go is a "common" operation that with the current API
> users are force to do a) manually and b) in quadratic time.
> Moreover, some layouts have a strides() member, others don't.
> It would be easier if they all had one :)
>
I think requiring them for all mappings (including user-defined ones)
is no go. However, we could add them for standard defined ones, however
That is a design change and would require a separate committee paper.
>
> >
> >>
> >> Do we need a member strides() for all strided layout mappings to avoid
> >> the issue?
>
We cannot add them, if the standard does not specify them.
> >>
> >> ---------------------------------------------------------
> >>
> >> The left-padded conversion rules are unexpected in two ways:
> >>
> >> 1. It allows conversion between mappings that have extent_types that
> >> aren't convertable (because it's not guaranteed at compile-time that the
> >> conversion is safe). AFAICT, this is covered by LWG4272 [1].
> >> [1]: https://cplusplus.github.io/LWG/issue4272
> >
> > Responding to the issue thread.
> >
> >>
> >>
> >> 2. The rules for converting between left-padded layouts with different
> >> padding values is different. For extents the rule is to allow sta ->
> >> sta, sta -> dyn and dyn -> dyn; but prevent dyn -> sta (because it can't
> >> be guaranteed at compile-time that the dynamic padding values matches
> >> the dynamic padding value match). For padding values we also disallow
> >> conversion for dyn -> dyn.
> >
> > Yah, I think we may want to allow this.
> >
>
> When's smartest to start working on proposals? Now or after
> we also have righpad so we can test everything. I'm leaning
> towards after rightpad.
>
I would say after rightpad, so we can have full implementation experience
and see the impact on the extensive test suite you have added.
>
> >
> >
> >> ---------------------------------------------------------
> >>
> >> If the padding value is statically 0 or 1, then it's effectively never
> >> padded, because:
> >>
> >> least_multiple_as_least(padding_value, extent(0)) == extent(0)
> >>
> >> either because of a special case (for padding_value == 0), or because `n
> >> % 1 == 0` and `n / 1 == n` (for padding_value == 1). Put differently,
> >> it's always exhaustive. However, is_always_exhaustive isn't true if
> >> exts.extent(0) is a dynamic extent. Even though that has no impact for
> >> padding_value <= 1.
> >>
> > The revenant mapping requirements are specified to allow false negative
> > results,
> > i.e. is_always_exhauystive may return false, even if the result would be
> > true.
> > We even have a note explaing that:
> > https://eel.is/c++draft/mdspan.layout#reqmts-note-1
> > This is to allow user-defined mapping to take simpler implementations,
> and
> > still be conforming.
> >
> > I think for the standard mapping we should however, be as precise as we
> > can, and I think
> > adding this special case makes sense for both padded layouts.
> >
>
> I'm aware and agree: a standardized mapping might as well be
> sharp, especially if it comes at no cost at runtime. (It might
> even reduce it by an imperceptably small amount.)
>
> >
> >
> >>
> >> ---------------------------------------------------------
> >>
> >> It's not possible to round-trip: layout_right -> layout_left_padded ->
> >> layout_right even for rank <= 1 where usually the difference between
> >> different mappings is ignored.
> >>
> > I think we should also keep that property. I think this will be touching
> > the same wording
> > as point 2, so we should handle them together.
> >
> > However, I would get all layouts implemented first, so we can test if we
> > got ever case right.
> >
>
> Same as above: we go for these after rightpad?
Agreed.
> >
> >>
> >> ---------------------------------------------------------
> >>
> >> There's similar problems with operator== as noted before; and for which
> >> I promised to (and never did) report a non-editorial issue.
> >>
> > I think it also makes sense to report issue with PR, once we can test all
> > mappings.
> >
>
> Makes sense: again after rightpad would be best?
>
Agreed.
>
> >
> >>
> >> --- 8< ---
> >>
> >> This commit adds a new layout layout_left_padded as standardized in
> >> N5014 but with one deviation. This includes checking of mandates and
> >> prerequisites, with exception of "slow" prerequisites, i.e. those that
> >> requires a for loop. It adds the feature testing macro submdspan as a
> >> purely internal macro and registers layout_left_padded in the std
> >> module.
> >>
> >> The standard mandates that the padding_value is representable as
> >> index_type. However, padding_value can be dynamic_extent which is
> >> defined as `size_t(-1)`. Hence, it's impossible to use a dynamic
> >> padding_value if index_type is int. The deviation is to only require
> >> static padding values to be representable as index_type; and thereby
> >> allow dynamic padding values for every index_type.
> >>
> >> libstdc++-v3/ChangeLog:
> >>
> >> * include/bits/version.def (submdspan): Add as internal
> >> only.
> >> * include/bits/version.h: Regenerate.
> >> * include/std/mdspan (__fwd_prod): New overload.
> >> (layout_left_padded): Add declaration and implementation.
> >> (layout_right_padded): Add declaration only.
> >> (__is_left_padded_mapping): New concept.
> >> (__is_right_padded_mapping): Ditto.
> >> (__standardized_mapping): Recognize left and right padded
> >> mappings.
> >> (layout_left::mapping::mapping): New overload for left
> >> padded mappings.
> >> * src/c++23/std.cc.in (layout_left_padded): Add.
> >> * testsuite/23_containers/mdspan/layouts/ctors.cc: Add tests
> for
> >> layout_left_padded.
> >> * testsuite/23_containers/mdspan/layouts/empty.cc: Ditto.
> >> * testsuite/23_containers/mdspan/layouts/mapping.cc: Ditto.
> >> * testsuite/23_containers/mdspan/layouts/debug/padded_neg.cc:
> New
> >> test.
> >> * testsuite/23_containers/mdspan/layouts/padded.cc: New test.
> >> * testsuite/23_containers/mdspan/layouts/padded_neg.cc: New
> test.
> >>
> >> Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
> >> ---
> >> libstdc++-v3/include/bits/version.def | 9 +
> >> libstdc++-v3/include/bits/version.h | 9 +
> >> libstdc++-v3/include/std/mdspan | 416 +++++++++++++-
> >> libstdc++-v3/src/c++23/std.cc.in | 8 +-
> >> .../23_containers/mdspan/layouts/ctors.cc | 61 +-
> >> .../mdspan/layouts/debug/padded_neg.cc | 3 +
> >> .../23_containers/mdspan/layouts/empty.cc | 10 +-
> >> .../23_containers/mdspan/layouts/mapping.cc | 198 +++++--
> >> .../23_containers/mdspan/layouts/padded.cc | 537 ++++++++++++++++++
> >> .../mdspan/layouts/padded_neg.cc | 225 ++++++++
> >> 10 files changed, 1414 insertions(+), 62 deletions(-)
> >> create mode 100644
> >> libstdc++-v3/testsuite/23_containers/mdspan/layouts/debug/padded_neg.cc
> >> create mode 100644
> >> libstdc++-v3/testsuite/23_containers/mdspan/layouts/padded.cc
> >> create mode 100644
> >> libstdc++-v3/testsuite/23_containers/mdspan/layouts/padded_neg.cc
> >>
> >> diff --git a/libstdc++-v3/include/bits/version.def
> >> b/libstdc++-v3/include/bits/version.def
> >> index 65b9a278776..1895078210a 100644
> >> --- a/libstdc++-v3/include/bits/version.def
> >> +++ b/libstdc++-v3/include/bits/version.def
> >> @@ -1050,6 +1050,15 @@ ftms = {
> >> cxxmin = 26;
> >> extra_cond = "__glibcxx_assume_aligned "
> >> "&& __glibcxx_is_sufficiently_aligned";
> >> + };
> >> +};
> >> +
> >> +ftms = {
> >> + name = submdspan;
> >> + no_stdname = true; // FIXME: remove
> >> + values = {
> >> + v = 202306;
> >> + cxxmin = 26;
> >> };
> >> };
> >>
> >> diff --git a/libstdc++-v3/include/bits/version.h
> >> b/libstdc++-v3/include/bits/version.h
> >> index b05249857d2..c6de494da08 100644
> >> --- a/libstdc++-v3/include/bits/version.h
> >> +++ b/libstdc++-v3/include/bits/version.h
> >> @@ -1178,6 +1178,15 @@
> >> #endif /* !defined(__cpp_lib_aligned_accessor) &&
> >> defined(__glibcxx_want_aligned_accessor) */
> >> #undef __glibcxx_want_aligned_accessor
> >>
> >> +#if !defined(__cpp_lib_submdspan)
> >> +# if (__cplusplus > 202302L)
> >> +# define __glibcxx_submdspan 202306L
> >> +# if defined(__glibcxx_want_all) || defined(__glibcxx_want_submdspan)
> >> +# endif
> >> +# endif
> >> +#endif /* !defined(__cpp_lib_submdspan) &&
> >> defined(__glibcxx_want_submdspan) */
> >> +#undef __glibcxx_want_submdspan
> >> +
> >> #if !defined(__cpp_lib_ssize)
> >> # if (__cplusplus >= 202002L)
> >> # define __glibcxx_ssize 201902L
> >> diff --git a/libstdc++-v3/include/std/mdspan
> >> b/libstdc++-v3/include/std/mdspan
> >> index 678b2619ebe..1270640ab6b 100644
> >> --- a/libstdc++-v3/include/std/mdspan
> >> +++ b/libstdc++-v3/include/std/mdspan
> >> @@ -44,6 +44,7 @@
> >>
> >> #define __glibcxx_want_mdspan
> >> #define __glibcxx_want_aligned_accessor
> >> +#define __glibcxx_want_submdspan
> >> #include <bits/version.h>
> >>
> >> #ifdef __glibcxx_mdspan
> >> @@ -476,6 +477,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> }
> >>
> >> // Preconditions: _r < _Extents::rank()
> >> + template<typename _Extents>
> >> + constexpr typename _Extents::index_type
> >> + __fwd_prod(const _Extents& __exts, size_t __begin, size_t __end)
> >> noexcept
> >> + {
> >> + size_t __sta_prod = [__begin, __end] {
> >> + span<const size_t> __sta_exts = __static_extents<_Extents>();
> >> + size_t __ret = 1;
> >> + for(auto __ext : __sta_exts.subspan(__begin, __end - __begin))
> >> + if (__ext != dynamic_extent)
> >> + __ret *= __ext;
> >> + return __ret;
> >> + }();
> >> + return __extents_prod(__exts, __sta_prod, __begin, __end);
> >> + }
> >> +
> >> template<typename _Extents>
> >> constexpr typename _Extents::index_type
> >> __fwd_prod(const _Extents& __exts, size_t __r) noexcept
> >> @@ -567,6 +583,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> class mapping;
> >> };
> >>
> >> +#ifdef __glibcxx_submdspan
> >> + template<size_t _PaddingValue>
> >> + struct layout_left_padded
> >> + {
> >> + template<typename _Extents>
> >> + class mapping;
> >> + };
> >> +
> >> + template<size_t _PaddingValue>
> >> + struct layout_right_padded
> >> + {
> >> + template<typename _Extents>
> >> + class mapping;
> >> + };
> >> +#endif
> >> +
> >> namespace __mdspan
> >> {
> >> template<typename _Tp>
> >> @@ -660,10 +692,42 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> is_same_v<typename _Layout::template mapping<typename
> >> _Mapping::extents_type>,
> >> _Mapping>;
> >>
> >> + template<template<size_t> typename _Layout, typename _Mapping>
> >> + concept __is_padded_mapping_of = requires
> >> + {
> >> + typename _Mapping::extents_type;
> >> + { _Mapping::padding_value } -> same_as<const size_t&>;
> >> + }
> >> + && same_as<_Mapping, typename
> >> _Layout<_Mapping::padding_value>::mapping<
> >> + typename _Mapping::extents_type>>;
> >> +
> >> +#ifdef __glibcxx_submdspan
> >> + template<typename _Mapping>
> >> + constexpr bool __is_left_padded_mapping = __is_padded_mapping_of<
> >> + layout_left_padded, _Mapping>;
> >> +
> >> + template<typename _Mapping>
> >> + constexpr bool __is_right_padded_mapping =
> __is_padded_mapping_of<
> >> + layout_right_padded, _Mapping>;
> >> +#else
> >> + template<typename _Mapping>
> >> + constexpr bool __is_left_padded_mapping = false;
> >> +
> >> + template<typename _Mapping>
> >> + constexpr bool __is_right_padded_mapping = false;
> >> +#endif
> >> +
> >> + template<typename _PaddedMapping>
> >> + constexpr size_t
> >> + __get_static_stride()
> >> + { return _PaddedMapping::_S_static_stride; }
> >> +
> >> template<typename _Mapping>
> >> concept __standardized_mapping = __mapping_of<layout_left,
> _Mapping>
> >> || __mapping_of<layout_right,
> >> _Mapping>
> >> - || __mapping_of<layout_stride,
> >> _Mapping>;
> >> + || __mapping_of<layout_stride,
> >> _Mapping>
> >> + ||
> >> __is_left_padded_mapping<_Mapping>
> >> + ||
> >> __is_right_padded_mapping<_Mapping>;
> >>
> >> // A tag type to create internal ctors.
> >> class __internal_ctor
> >> @@ -717,6 +781,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> : mapping(__other.extents(), __mdspan::__internal_ctor{})
> >> { __glibcxx_assert(*this == __other); }
> >>
> >> + template<class _LeftPaddedMapping>
> >> + requires __mdspan::__is_left_padded_mapping<_LeftPaddedMapping>
> >> + && is_constructible_v<extents_type,
> >> + typename
> >> _LeftPaddedMapping::extents_type>
> >> + constexpr
> >> + explicit(!is_convertible_v<typename
> >> _LeftPaddedMapping::extents_type,
> >> + extents_type>)
> >> + mapping(const _LeftPaddedMapping& __other) noexcept
> >> + : mapping(__other.extents(), __mdspan::__internal_ctor{})
> >> + {
> >> + constexpr size_t __ostride_sta =
> __mdspan::__get_static_stride<
> >> + _LeftPaddedMapping>();
> >> +
> >> + if constexpr (extents_type::rank() > 1
> >> + && extents_type::static_extent(0) != dynamic_extent
> >> + && __ostride_sta != dynamic_extent)
> >> + static_assert(extents_type::static_extent(0) ==
> __ostride_sta);
> >> +
> >> + __glibcxx_assert(extents_type::rank() <= 1
> >> + || __other.stride(1) == __other.extents().extent(0));
> >> + }
> >> +
> >> constexpr mapping&
> >> operator=(const mapping&) noexcept = default;
> >>
> >> @@ -1164,6 +1250,334 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> [[no_unique_address]] _S_strides_t _M_strides;
> >> };
> >>
> >> +#ifdef __glibcxx_submdspan
> >> + namespace __mdspan
> >> + {
> >> + template<typename _Io, typename _I1, typename _I2>
> >> + constexpr _Io
> >> + __least_multiple_at_least(_I1 __x, _I2 __y)
> >> + {
> >> + if (__x == 0)
> >> + return static_cast<_Io>(__y);
> >> + return static_cast<_Io>((__y / __x + (__y % __x != 0)) * __x);
> >> + }
> >> +
> >> + template<typename _Extents, typename _Stride, typename... _Indices>
> >> + constexpr typename _Extents::index_type
> >> + __linear_index_leftpad(const _Extents& __exts, _Stride __stride,
> >> + _Indices... __indices)
> >> + {
> >> + // i0 + stride*(i1 + extents.extent(1)*...)
> >> + using _IndexType = typename _Extents::index_type;
> >> + _IndexType __res = 0;
> >> + if constexpr (sizeof...(__indices) > 0)
> >> + {
> >> + _IndexType __mult = 1;
> >> +
> >> + auto __update_rest = [&, __pos = 1u](_IndexType __idx)
> mutable
> >> + {
> >> + __res += __idx * __mult;
> >> + __mult *= __exts.extent(__pos);
> >> + ++__pos;
> >> + };
> >> +
> >> + auto __update = [&](_IndexType __idx, auto... __rest)
> >> + {
> >> + __res += __idx;
> >> + __mult = __stride.extent(0);
> >> + (__update_rest(__rest), ...);
> >> + };
> >> + __update(__indices...);
> >> + }
> >> + return __res;
> >> + }
> >> + }
> >> +
> >> + template<size_t _PaddingValue>
> >> + template<typename _Extents>
> >> + class layout_left_padded<_PaddingValue>::mapping
> >> + {
> >> + public:
> >> + static constexpr size_t padding_value = _PaddingValue;
> >> +
> >> + using extents_type = _Extents;
> >> + using index_type = typename extents_type::index_type;
> >> + using size_type = typename extents_type::size_type;
> >> + using rank_type = typename extents_type::rank_type;
> >> + using layout_type = layout_left_padded<padding_value>;
> >> +
> >> + static_assert(__mdspan::__representable_size<extents_type,
> >> index_type>,
> >> + "The size of extents_type must be representable as
> index_type");
> >> +
> >> + // DEVIATION the standard requires that _PaddingValue is
> >> representable
> >> + // as index_type. This seems unlikely, because if index_type <
> >> size_t
> >> + // then the mandate is violated for `padding_value ==
> >> dynamic_extent`.
> >> + // Which means that padding_value == dynamic_extent only
> reliably
> >> work
> >> + // if index_type == size_t (e.g. it fails for index_type == int
> on
> >> a
> >> + // 64bit system).
> >> + static_assert((padding_value == dynamic_extent)
> >> + || (padding_value <= numeric_limits<index_type>::max()),
> >> + "padding_value must be representable as index_type");
> >> +
> >> + private:
> >> + static constexpr size_t _S_rank = _Extents::rank();
> >> + static constexpr size_t _S_static_stride = [] consteval
> >> + {
> >> + if constexpr (_S_rank <= 1)
> >> + return size_t(0);
> >> + else if constexpr (padding_value == dynamic_extent ||
> >> + _Extents::static_extent(0) == dynamic_extent)
> >> + return dynamic_extent;
> >> + else
> >> + return
> >> __mdspan::__least_multiple_at_least<size_t>(padding_value,
> >> + size_t(_Extents::static_extent(0)));
> >> + }();
> >> +
> >> + constexpr friend size_t
> >> + __mdspan::__get_static_stride<mapping>();
> >> +
> >> + public:
> >> + constexpr
> >> + mapping() noexcept
> >> + : mapping(extents_type{})
> >> + { }
> >> +
> >> + constexpr
> >> + mapping(const mapping&) noexcept = default;
> >> +
> >> + constexpr
> >> + mapping(const extents_type& __exts)
> >> + : _M_extents(__exts)
> >> + {
> >> +
> >> __glibcxx_assert(__mdspan::__is_representable_extents(_M_extents));
> >> + if (_S_rank > 1)
> >> + {
> >> + index_type __stride;
> >> + if constexpr (padding_value == dynamic_extent)
> >> + __stride = __exts.extent(0);
> >> + else
> >> + __stride =
> __mdspan::__least_multiple_at_least<index_type>(
> >> + padding_value, __exts.extent(0));
> >> +
> >> + _M_stride = std::extents<index_type,
> >> _S_static_stride>{__stride};
> >> + }
> >> + }
> >> +
> >> + template<__mdspan::__valid_index_type<index_type> _OIndexType>
> >> + constexpr mapping(const extents_type& __exts, _OIndexType
> __opad)
> >> + : _M_extents(__exts)
> >> + {
> >> + if constexpr (std::is_integral_v<_OIndexType>)
> >> + {
> >> + __glibcxx_assert(cmp_less_equal(__opad,
> >> + numeric_limits<index_type>::max()));
> >> + if constexpr (std::is_signed_v<_OIndexType>)
> >> + __glibcxx_assert(__opad >= 0);
> >> + }
> >> +
> >> + auto __pad = static_cast<index_type>(std::move(__opad));
> >> + if constexpr (std::is_signed_v<index_type>)
> >> + __glibcxx_assert(__pad >= 0);
> >> + if constexpr (padding_value != dynamic_extent)
> >> + __glibcxx_assert(cmp_equal(padding_value, __pad));
> >> + if constexpr (_S_rank > 1)
> >> + _M_stride = _S_stride_type{
> >> + __mdspan::__least_multiple_at_least<index_type>(__pad,
> >> + __exts.extent(0))};
> >> + }
> >> +
> >> + template<typename _OExtents>
> >> + requires is_constructible_v<extents_type, _OExtents>
> >> + constexpr explicit(!is_convertible_v<_OExtents, extents_type>)
> >> + mapping(const layout_left::mapping<_OExtents>& __other)
> >> + : mapping(extents_type(__other.extents()))
> >> + {
> >> + if constexpr (_OExtents::rank() > 1)
> >> + static_assert(_S_static_stride == dynamic_extent
> >> + || _OExtents::static_extent(0) == dynamic_extent
> >> + || _S_static_stride == _OExtents::static_extent(0),
> >> + "(*this).stride(1) must be compatible with
> >> other.stride(1)");
> >> +
> >> + // REVIEW: can this one go, because we check
> >> + // __is_representable_extents in mapping(const mapping&)
> >> +
> __glibcxx_assert(cmp_less_equal(__other.required_span_size(),
> >> +
> >> numeric_limits<index_type>::max()));
> >> + __glibcxx_assert((!(_S_rank > 1 && padding_value !=
> >> dynamic_extent))
> >> + || (_M_stride.extent(0) == __other.extents().extent(0)));
> >> + }
> >> +
> >> + template<typename _OExtents>
> >> + requires is_constructible_v<_OExtents, extents_type>
> >> + constexpr explicit(_OExtents::rank() > 0)
> >> + mapping(const typename layout_stride::mapping<_OExtents>&
> >> __other)
> >> + : _M_extents(__other.extents())
> >> + {
> >> + if constexpr (_S_rank > 1 && padding_value !=
> dynamic_extent)
> >> + __glibcxx_assert(cmp_equal(__other.stride(1),
> >> +
> >> __mdspan::__least_multiple_at_least<index_type>(padding_value,
> >> + __other.extents().extent(0))));
> >> +
> >> + if constexpr (_S_rank > 0)
> >> + __glibcxx_assert(__other.stride(0) == 1);
> >> +
> >> +
> __glibcxx_assert(cmp_less_equal(__other.required_span_size(),
> >> +
> >> numeric_limits<index_type>::max()));
> >> +
> >> + if constexpr (_S_rank > 1 && _S_static_stride ==
> >> dynamic_extent)
> >> + _M_stride = _S_stride_type{__other.stride(1)};
> >> + }
> >> +
> >> + template<typename _LeftPaddedMapping>
> >> + requires
> __mdspan::__is_left_padded_mapping<_LeftPaddedMapping>
> >> + && is_constructible_v<extents_type,
> >> + typename
> >> _LeftPaddedMapping::extents_type>
> >> + constexpr explicit(_S_rank > 1 && (padding_value !=
> >> dynamic_extent
> >> + || _LeftPaddedMapping::padding_value == dynamic_extent))
> >> + mapping(const _LeftPaddedMapping& __other)
> >> + : _M_extents(__other.extents())
> >> + {
> >> + if constexpr (_S_rank > 1)
> >> + {
> >> + static_assert(padding_value == dynamic_extent
> >> + || _LeftPaddedMapping::padding_value == dynamic_extent
> >> + || padding_value == _LeftPaddedMapping::padding_value,
> >> + "If neither padding_value is dynamic_extent, then they
> >> must "
> >> + "be equal");
> >> +
> >> + __glibcxx_assert((padding_value == dynamic_extent
> >> + || cmp_equal(__other.stride(1),
> >> + __mdspan::__least_multiple_at_least<index_type>(
> >> + padding_value, __other.extents().extent(0))))
> >> + && "other.stride(1) must be the next larger multiple of
> "
> >> + "padding_value that is greater or equal to
> >> other.extent(0)");
> >> + }
> >> +
> __glibcxx_assert(cmp_less_equal(__other.required_span_size(),
> >> + numeric_limits<index_type>::max()));
> >> +
> >> + if constexpr (_S_rank > 1 && (_S_static_stride ==
> >> dynamic_extent))
> >> + _M_stride = _S_stride_type{__other.stride(1)};
> >> + }
> >> +
> >> + template<typename _RightPaddedMapping>
> >> + requires
> >> (__mdspan::__is_right_padded_mapping<_RightPaddedMapping>
> >> + || __mdspan::__mapping_of<layout_right,
> _RightPaddedMapping>)
> >> + && (_S_rank <= 1)
> >> + && is_constructible_v<extents_type,
> >> + typename
> >> _RightPaddedMapping::extents_type>
> >> + constexpr explicit(!is_convertible_v<
> >> + typename _RightPaddedMapping::extents_type, extents_type>)
> >> + mapping(const _RightPaddedMapping& __other) noexcept
> >> + : _M_extents(__other.extents())
> >> + {
> >> +
> __glibcxx_assert(cmp_less_equal(__other.required_span_size(),
> >> + numeric_limits<index_type>::max()));
> >> + }
> >> +
> >> + constexpr mapping&
> >> + operator=(const mapping&) noexcept = default;
> >> +
> >> + constexpr const extents_type&
> >> + extents() const noexcept { return _M_extents; }
> >> +
> >> + constexpr array<index_type, _S_rank>
> >> + strides() const noexcept
> >> + {
> >> + array<index_type, _S_rank> __ret;
> >> + if constexpr (_S_rank > 0)
> >> + __ret[0] = 1;
> >> + if constexpr (_S_rank > 1)
> >> + __ret[1] = _M_stride.extent(0);
> >> + if constexpr (_S_rank > 2)
> >> + for(size_t __i = 2; __i < _S_rank; ++__i)
> >> + __ret[__i] = __ret[__i - 1] * _M_extents.extent(__i - 1);
> >> + return __ret;
> >> + }
> >> +
> >> + constexpr index_type
> >> + required_span_size() const noexcept
> >> + {
> >> + if constexpr (_S_rank == 0)
> >> + return 1;
> >> + else if (__mdspan::__empty(_M_extents))
> >> + return 0;
> >> + else
> >> + return _M_stride.extent(0) *
> __mdspan::__rev_prod(_M_extents,
> >> 0)
> >> + - (_M_stride.extent(0) - _M_extents.extent(0));
> >> + }
> >> +
> >> + // _GLIBCXX_RESOLVE_LIB_DEFECTS
> >> + // 4314. Missing move in mdspan layout mapping::operator()
> >> + template<__mdspan::__valid_index_type<index_type>... _Indices>
> >> + requires (sizeof...(_Indices) == _S_rank)
> >> + constexpr index_type
> >> + operator()(_Indices... __indices) const noexcept
> >> + {
> >> + return __mdspan::__linear_index_leftpad(_M_extents,
> _M_stride,
> >> + static_cast<index_type>(std::move(__indices))...);
> >> + }
> >> +
> >> + static constexpr bool
> >> + is_always_unique() noexcept { return true; }
> >> +
> >> + static constexpr bool
> >> + is_always_exhaustive() noexcept
> >> + {
> >> + if constexpr (_S_rank <= 1)
> >> + return true;
> >> + else
> >> + return extents_type::static_extent(0) != dynamic_extent
> >> + && _S_static_stride != dynamic_extent
> >> + && extents_type::static_extent(0) ==
> _S_static_stride;
> >> + }
> >> +
> >> + static constexpr bool
> >> + is_always_strided() noexcept { return true; }
> >> +
> >> + static constexpr bool
> >> + is_unique() noexcept { return true; }
> >> +
> >> + constexpr bool
> >> + is_exhaustive() const noexcept
> >> + {
> >> + if constexpr (is_always_exhaustive())
> >> + return true;
> >> + else
> >> + return _M_extents.extent(0) == _M_stride.extent(0);
> >> + }
> >> +
> >> + static constexpr bool
> >> + is_strided() noexcept { return true; }
> >> +
> >> + constexpr index_type
> >> + stride(rank_type __r) const noexcept
> >> + {
> >> + __glibcxx_assert(__r < _S_rank);
> >> + if (__r == 0)
> >> + return 1;
> >> + else
> >> + return static_cast<index_type>(
> >> + static_cast<size_t>(_M_stride.extent(0)) *
> >> + static_cast<size_t>(__mdspan::__fwd_prod(_M_extents, 1,
> >> __r)));
> >> + }
> >> +
> >> + template<typename _LeftPaddedMapping>
> >> +
> requires(__mdspan::__is_left_padded_mapping<_LeftPaddedMapping>
> >> + && _LeftPaddedMapping::extents_type::rank() ==
> _S_rank)
> >> + friend constexpr bool
> >> + operator==(const mapping& __self, const _LeftPaddedMapping&
> >> __other)
> >> + noexcept
> >> + {
> >> + return __self.extents() == __other.extents()
> >> + && (_S_rank < 2 || __self.stride(1) == __other.stride(1));
> >> + }
> >> +
> >> + private:
> >> + using _S_stride_type = std::extents<index_type,
> _S_static_stride>;
> >> + [[no_unique_address]] _S_stride_type _M_stride;
> >> + [[no_unique_address]] extents_type _M_extents{};
> >> + };
> >> +#endif
> >> +
> >> template<typename _ElementType>
> >> struct default_accessor
> >> {
> >> diff --git a/libstdc++-v3/src/c++23/std.cc.in b/libstdc++-v3/src/c++23/
> >> std.cc.in
> >> index a217a87330b..c2854cb1828 100644
> >> --- a/libstdc++-v3/src/c++23/std.cc.in
> >> +++ b/libstdc++-v3/src/c++23/std.cc.in
> >> @@ -1869,9 +1869,11 @@ export namespace std
> >> using std::aligned_accessor;
> >> #endif
> >> using std::mdspan;
> >> - // FIXME layout_left_padded, layout_right_padded, strided_slice,
> >> - // submdspan_mapping_result, full_extent_t, full_extent,
> >> submdspan_extents,
> >> - // mdsubspan
> >> +#if __glibcxx_submdspan
> >> + using std::layout_left_padded;
> >> +#endif
> >> + // FIXME layout_right_padded, strided_slice,
> submdspan_mapping_result,
> >> + // full_extent_t, full_extent, submdspan_extents, mdsubspan
> >> }
> >> #endif
> >>
> >> diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/ctors.cc
> >> b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/ctors.cc
> >> index 23c0a55dae1..290c53647cf 100644
> >> --- a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/ctors.cc
> >> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/ctors.cc
> >> @@ -6,6 +6,16 @@
> >>
> >> constexpr size_t dyn = std::dynamic_extent;
> >>
> >> +template<typename Layout>
> >> + constexpr bool
> >> + is_layout_leftpad = false;
> >> +
> >> +#ifdef __glibcxx_submdspan
> >> +template<size_t Pad>
> >> + constexpr bool
> >> + is_layout_leftpad<std::layout_left_padded<Pad>> = true;
> >> +#endif
> >> +
> >> template<typename Mapping, typename IndexType, size_t... Extents>
> >> constexpr void
> >> verify(std::extents<IndexType, Extents...> oexts)
> >> @@ -27,7 +37,6 @@ template<typename Mapping, typename OMapping>
> >> VERIFY(std::cmp_equal(m.stride(i), other.stride(i)));
> >> }
> >>
> >> -
> >> template<typename To, typename From>
> >> constexpr void
> >> verify_convertible(From from)
> >> @@ -40,7 +49,10 @@ template<typename To, typename From>
> >> constexpr void
> >> verify_nothrow_convertible(From from)
> >> {
> >> - static_assert(std::is_nothrow_constructible_v<To, From>);
> >> + if constexpr (is_layout_leftpad<typename To::layout_type>)
> >> + static_assert(std::is_constructible_v<To, From>);
> >> + else
> >> + static_assert(std::is_nothrow_constructible_v<To, From>);
> >> verify_convertible<To>(from);
> >> }
> >>
> >> @@ -57,7 +69,10 @@ template<typename To, typename From>
> >> constexpr void
> >> verify_nothrow_constructible(From from)
> >> {
> >> - static_assert(std::is_nothrow_constructible_v<To, From>);
> >> + if constexpr (is_layout_leftpad<typename To::layout_type>)
> >> + static_assert(std::is_constructible_v<To, From>);
> >> + else
> >> + static_assert(std::is_nothrow_constructible_v<To, From>);
> >> verify_constructible<To>(from);
> >> }
> >>
> >> @@ -196,6 +211,16 @@ namespace from_extents
> >> // ctor: mapping(mapping<OExtents>)
> >> namespace from_same_layout
> >> {
> >> + template<typename Layout, typename Extents, typename OExtents>
> >> + constexpr void
> >> + verify_convertible(OExtents exts)
> >> + {
> >> + using Mapping = typename Layout::mapping<Extents>;
> >> + using OMapping = typename Layout::mapping<OExtents>;
> >> +
> >> + ::verify_convertible<Mapping>(OMapping(exts));
> >> + }
> >> +
> >> template<typename Layout, typename Extents, typename OExtents>
> >> constexpr void
> >> verify_nothrow_convertible(OExtents exts)
> >> @@ -223,8 +248,12 @@ namespace from_same_layout
> >> verify_nothrow_convertible<Layout, std::extents<unsigned int>>(
> >> std::extents<int>{});
> >>
> >> - verify_nothrow_constructible<Layout, std::extents<int>>(
> >> - std::extents<unsigned int>{});
> >> + if constexpr (!is_layout_leftpad<Layout>)
> >> + verify_nothrow_constructible<Layout, std::extents<int>>(
> >> + std::extents<unsigned int>{});
> >> + else
> >> + verify_convertible<Layout, std::extents<int>>(
> >> + std::extents<unsigned int>{});
> >>
> >> assert_not_constructible<
> >> typename Layout::mapping<std::extents<int>>,
> >> @@ -234,8 +263,12 @@ namespace from_same_layout
> >> typename Layout::mapping<std::extents<int, 1>>,
> >> typename Layout::mapping<std::extents<int>>>();
> >>
> >> - verify_nothrow_constructible<Layout, std::extents<int, 1>>(
> >> - std::extents<int, dyn>{1});
> >> + if constexpr (!is_layout_leftpad<Layout>)
> >> + verify_nothrow_constructible<Layout, std::extents<int, 1>>(
> >> + std::extents<int, dyn>{1});
> >> + else
> >> + verify_convertible<Layout, std::extents<int, 1>>(
> >> + std::extents<int, dyn>{1});
> >>
> >> verify_nothrow_convertible<Layout, std::extents<int, dyn>>(
> >> std::extents<int, 1>{});
> >> @@ -247,8 +280,12 @@ namespace from_same_layout
> >> verify_nothrow_constructible<Layout, std::extents<int, 1, 2>>(
> >> std::extents<int, dyn, 2>{1});
> >>
> >> - verify_nothrow_convertible<Layout, std::extents<int, dyn, 2>>(
> >> - std::extents<int, 1, 2>{});
> >> + if constexpr (!is_layout_leftpad<Layout>)
> >> + verify_nothrow_convertible<Layout, std::extents<int, dyn, 2>>(
> >> + std::extents<int, 1, 2>{});
> >> + else
> >> + verify_nothrow_constructible<Layout, std::extents<int, dyn, 2>>(
> >> + std::extents<int, 1, 2>{});
> >> return true;
> >> }
> >>
> >> @@ -429,6 +466,12 @@ main()
> >> {
> >> test_all<std::layout_left>();
> >> test_all<std::layout_right>();
> >> +#ifdef __glibcxx_submdspan
> >> + test_all<std::layout_left_padded<0>>();
> >> + test_all<std::layout_left_padded<1>>();
> >> + test_all<std::layout_left_padded<2>>();
> >> + test_all<std::layout_left_padded<dyn>>();
> >> +#endif
> >>
> >> from_left_or_right::test_all<std::layout_left, std::layout_right>();
> >> from_left_or_right::test_all<std::layout_right, std::layout_left>();
> >> diff --git
> >>
> a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/debug/padded_neg.cc
> >>
> b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/debug/padded_neg.cc
> >> new file mode 100644
> >> index 00000000000..d4f9003fba6
> >> --- /dev/null
> >> +++
> >>
> b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/debug/padded_neg.cc
> >> @@ -0,0 +1,3 @@
> >> +// { dg-do compile { target c++26 } }
> >> +// { dg-require-debug-mode "" }
> >> +#include<mdspan>
> >> diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/empty.cc
> >> b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/empty.cc
> >> index 655b9b6d6c3..d1aff172b64 100644
> >> --- a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/empty.cc
> >> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/empty.cc
> >> @@ -35,7 +35,8 @@ test_static_overflow()
> >> {
> >> constexpr Int n1 = std::numeric_limits<Int>::max();
> >> constexpr size_t n2 = std::dynamic_extent - 1;
> >> - constexpr size_t n = std::cmp_less(n1, n2) ? size_t(n1) : n2;
> >> + // Allow some room for padding.
> >> + constexpr size_t n = (std::cmp_less(n1, n2) ? size_t(n1) : n2) - 4;
> >>
> >> verify_all(typename Layout::mapping<std::extents<Int, n, n, 0, n,
> >> n>>{});
> >> verify_all(typename Layout::mapping<std::extents<Int, 0, n, n,
> n>>{});
> >> @@ -73,7 +74,8 @@ test_dynamic_overflow()
> >> {
> >> constexpr Int n1 = std::numeric_limits<Int>::max();
> >> constexpr size_t n2 = std::dynamic_extent - 1;
> >> - constexpr Int n = std::cmp_less(n1, n2) ? n1 : Int(n2);
> >> + // Allow some room for padding.
> >> + constexpr Int n = (std::cmp_less(n1, n2) ? n1 : Int(n2)) - 4;
> >>
> >> verify_all(make_mapping<Layout>(
> >> std::extents<Int, dyn, dyn, 0, dyn, dyn>{n, n, n, n}));
> >> @@ -127,5 +129,9 @@ main()
> >> static_assert(test_all<std::layout_left>());
> >> static_assert(test_all<std::layout_right>());
> >> static_assert(test_all<std::layout_stride>());
> >> +#ifdef __glibcxx_submdspan
> >> + static_assert(test_all<std::layout_left_padded<1>>());
> >> + static_assert(test_all<std::layout_left_padded<2>>());
> >> +#endif
> >> return 0;
> >> }
> >> diff --git
> >> a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/mapping.cc
> >> b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/mapping.cc
> >> index 58bce514435..3b9719eaf5d 100644
> >> --- a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/mapping.cc
> >> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/mapping.cc
> >> @@ -7,6 +7,15 @@
> >>
> >> constexpr size_t dyn = std::dynamic_extent;
> >>
> >> +template<typename Mapping>
> >> + concept has_static_is_exhaustive = requires
> >> + {
> >> + { Mapping::is_exhaustive() } -> std::same_as<bool>;
> >> + };
> >> +
> >>
> >>
> +static_assert(has_static_is_exhaustive<std::layout_right::mapping<std::extents<int>>>);
> >>
> >>
> +static_assert(!has_static_is_exhaustive<std::layout_stride::mapping<std::extents<int>>>);
> >> +
> >> template<typename Layout, typename Extents>
> >> constexpr bool
> >> test_mapping_properties()
> >> @@ -32,7 +41,7 @@ template<typename Layout, typename Extents>
> >>
> >> static_assert(M::is_always_unique() && M::is_unique());
> >> static_assert(M::is_always_strided() && M::is_strided());
> >> - if constexpr (!std::is_same_v<Layout, std::layout_stride>)
> >> + if constexpr (has_static_is_exhaustive<M>)
> >> static_assert(M::is_always_exhaustive() && M::is_exhaustive());
> >> return true;
> >> }
> >> @@ -306,7 +315,7 @@ template<typename Layout>
> >> constexpr void
> >> test_stride_1d()
> >> {
> >> - std::layout_left::mapping<std::extents<int, 3>> m;
> >> + typename Layout::mapping<std::extents<int, 3>> m;
> >> VERIFY(m.stride(0) == 1);
> >> }
> >>
> >> @@ -321,73 +330,150 @@ template<>
> >> }
> >>
> >> template<typename Layout>
> >> - constexpr void
> >> - test_stride_2d();
> >> +struct TestStride2D;
> >>
> >> template<>
> >> - constexpr void
> >> - test_stride_2d<std::layout_left>()
> >> + struct TestStride2D<std::layout_left>
> >> {
> >> - std::layout_left::mapping<std::extents<int, 3, 5>> m;
> >> - VERIFY(m.stride(0) == 1);
> >> - VERIFY(m.stride(1) == 3);
> >> - }
> >> + static constexpr void
> >> + run()
> >> + {
> >> + std::layout_left::mapping<std::extents<int, 3, 5>> m;
> >> + VERIFY(m.stride(0) == 1);
> >> + VERIFY(m.stride(1) == 3);
> >> + }
> >> + };
> >>
> >> template<>
> >> - constexpr void
> >> - test_stride_2d<std::layout_right>()
> >> + struct TestStride2D<std::layout_right>
> >> {
> >> - std::layout_right::mapping<std::extents<int, 3, 5>> m;
> >> - VERIFY(m.stride(0) == 5);
> >> - VERIFY(m.stride(1) == 1);
> >> - }
> >> + static constexpr void
> >> + run()
> >> + {
> >> + std::layout_right::mapping<std::extents<int, 3, 5>> m;
> >> + VERIFY(m.stride(0) == 5);
> >> + VERIFY(m.stride(1) == 1);
> >> + }
> >> + };
> >>
> >> template<>
> >> + struct TestStride2D<std::layout_stride>
> >> + {
> >> + static constexpr void
> >> + run()
> >> + {
> >> + std::array<int, 2> strides{13, 2};
> >> + std::layout_stride::mapping m(std::extents<int, 3, 5>{},
> strides);
> >> + VERIFY(m.stride(0) == strides[0]);
> >> + VERIFY(m.stride(1) == strides[1]);
> >> + VERIFY(m.strides() == strides);
> >> + }
> >> + };
> >> +
> >> +#if __glibcxx_submdspan
> >> +template<size_t PaddingValue>
> >> + struct TestStride2D<std::layout_left_padded<PaddingValue>>
> >> + {
> >> + static constexpr void
> >> + run()
> >> + {
> >> + using Layout = std::layout_left_padded<PaddingValue>;
> >> + typename Layout::mapping<std::extents<int, 3, 5>> m;
> >> +
> >> + VERIFY(m.stride(0) == 1);
> >> +
> >> + // The next multiple of PaddingValue, that's greater or equal
> >> + // to exts.extent(0) is the unique value in the range:
> >> + // [exts.extent(0), exts.extent(0) + PaddingValue)
> >> + // that is divisible by PaddingValue.
> >> + VERIFY((m.stride(1) % PaddingValue) == 0);
> >> + VERIFY(3 <= m.stride(1) && std::cmp_less(m.stride(1), 3 +
> >> PaddingValue));
> >> + }
> >> + };
> >> +#endif
> >> +
> >> +template<typename Layout>
> >> constexpr void
> >> - test_stride_2d<std::layout_stride>()
> >> + test_stride_2d()
> >> {
> >> - std::array<int, 2> strides{13, 2};
> >> - std::layout_stride::mapping m(std::extents<int, 3, 5>{}, strides);
> >> - VERIFY(m.stride(0) == strides[0]);
> >> - VERIFY(m.stride(1) == strides[1]);
> >> - VERIFY(m.strides() == strides);
> >> + TestStride2D<Layout>::run();
> >> }
> >>
> >> template<typename Layout>
> >> - constexpr void
> >> - test_stride_3d();
> >> +struct TestStride3D;
> >>
> >> template<>
> >> - constexpr void
> >> - test_stride_3d<std::layout_left>()
> >> + struct TestStride3D<std::layout_left>
> >> {
> >> - std::layout_left::mapping m(std::dextents<int, 3>(3, 5, 7));
> >> - VERIFY(m.stride(0) == 1);
> >> - VERIFY(m.stride(1) == 3);
> >> - VERIFY(m.stride(2) == 3*5);
> >> - }
> >> + static constexpr void
> >> + run()
> >> + {
> >> + std::layout_left::mapping m(std::dextents<int, 3>(3, 5, 7));
> >> + VERIFY(m.stride(0) == 1);
> >> + VERIFY(m.stride(1) == 3);
> >> + VERIFY(m.stride(2) == 3*5);
> >> + }
> >> + };
> >> +
> >>
> >> template<>
> >> - constexpr void
> >> - test_stride_3d<std::layout_right>()
> >> + struct TestStride3D<std::layout_right>
> >> {
> >> - std::layout_right::mapping m(std::dextents<int, 3>(3, 5, 7));
> >> - VERIFY(m.stride(0) == 5*7);
> >> - VERIFY(m.stride(1) == 7);
> >> - VERIFY(m.stride(2) == 1);
> >> - }
> >> + static constexpr void
> >> + run()
> >> + {
> >> + std::layout_right::mapping m(std::dextents<int, 3>(3, 5, 7));
> >> + VERIFY(m.stride(0) == 5*7);
> >> + VERIFY(m.stride(1) == 7);
> >> + VERIFY(m.stride(2) == 1);
> >> + }
> >> + };
> >>
> >> template<>
> >> + struct TestStride3D<std::layout_stride>
> >> + {
> >> + static constexpr void
> >> + run()
> >> + {
> >> + std::dextents<int, 3> exts(3, 5, 7);
> >> + std::array<int, 3> strides{11, 2, 41};
> >> + std::layout_stride::mapping<std::dextents<int, 3>> m(exts,
> strides);
> >> + VERIFY(m.stride(0) == strides[0]);
> >> + VERIFY(m.stride(1) == strides[1]);
> >> + VERIFY(m.stride(2) == strides[2]);
> >> + VERIFY(m.strides() == strides);
> >> + }
> >> + };
> >> +
> >> +#if __glibcxx_submdspan
> >> +template<size_t PaddingValue>
> >> + struct TestStride3D<std::layout_left_padded<PaddingValue>>
> >> + {
> >> + static constexpr void
> >> + run()
> >> + {
> >> + using Layout = std::layout_left_padded<PaddingValue>;
> >> + typename Layout::mapping<std::extents<int, 3, 5, 7>> m;
> >> +
> >> + VERIFY(m.stride(0) == 1);
> >> +
> >> + // The next multiple of PaddingValue, that's greater or equal
> >> + // to exts.extent(0) is the unique value in the range:
> >> + // [exts.extent(0), exts.extent(0) + PaddingValue)
> >> + // that is divisible by PaddingValue.
> >> + VERIFY((m.stride(1) % PaddingValue) == 0);
> >> + VERIFY(3 <= m.stride(1) && std::cmp_less(m.stride(1), 3 +
> >> PaddingValue));
> >> +
> >> + VERIFY(m.stride(1) * 5 == m.stride(2));
> >> + }
> >> + };
> >> +#endif
> >> +
> >> +template<typename Layout>
> >> constexpr void
> >> - test_stride_3d<std::layout_stride>()
> >> + test_stride_3d()
> >> {
> >> - std::dextents<int, 3> exts(3, 5, 7);
> >> - std::array<int, 3> strides{11, 2, 41};
> >> - std::layout_stride::mapping<std::dextents<int, 3>> m(exts,
> strides);
> >> - VERIFY(m.stride(0) == strides[0]);
> >> - VERIFY(m.stride(1) == strides[1]);
> >> - VERIFY(m.stride(2) == strides[2]);
> >> - VERIFY(m.strides() == strides);
> >> + TestStride3D<Layout>::run();
> >> }
> >>
> >> template<typename Layout>
> >> @@ -411,7 +497,8 @@ template<typename Layout>
> >> test_has_stride_0d()
> >> {
> >> using Mapping = typename Layout::mapping<std::extents<int>>;
> >> - constexpr bool expected = std::is_same_v<Layout,
> std::layout_stride>;
> >> + constexpr bool expected = !(std::is_same_v<Layout,
> std::layout_left>
> >> + || std::is_same_v<Layout, std::layout_right>);
> >> static_assert(has_stride<Mapping> == expected);
> >> }
> >>
> >> @@ -561,10 +648,27 @@ main()
> >> test_all<std::layout_left>();
> >> test_all<std::layout_right>();
> >> test_all<std::layout_stride>();
> >> +#ifdef __glibcxx_submdspan
> >> + test_all<std::layout_left_padded<1>>();
> >> + test_all<std::layout_left_padded<2>>();
> >> + test_all<std::layout_left_padded<5>>();
> >> +#endif
> >>
> >> test_has_op_eq<std::layout_right, std::layout_left, false>();
> >> test_has_op_eq<std::layout_right, std::layout_stride, true>();
> >> test_has_op_eq<std::layout_left, std::layout_stride, true>();
> >> +#ifdef __glibcxx_submdspan
> >> + test_has_op_eq<std::layout_left, std::layout_left_padded<0>,
> false>();
> >> + test_has_op_eq<std::layout_left, std::layout_left_padded<6>,
> false>();
> >> + test_has_op_eq<std::layout_left, std::layout_left_padded<dyn>,
> false>();
> >> + // REVIEW: The next one looks strange, because it's neither. Somehow,
> >> the
> >> + // conversion rules seem to be playing a critical role again.
> >> + // test_has_op_eq<std::layout_right, std::layout_left_padded<0>,
> >> false>();
> >> +
> >> + test_has_op_eq<std::layout_left_padded<2>,
> std::layout_left_padded<6>,
> >> true>();
> >> + test_has_op_eq<std::layout_left_padded<2>,
> >> std::layout_left_padded<dyn>, true>();
> >> +#endif
> >> +
> >> test_has_op_eq_peculiar();
> >> return 0;
> >> }
> >> diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/padded.cc
> >> b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/padded.cc
> >> new file mode 100644
> >> index 00000000000..6e79e191ff7
> >> --- /dev/null
> >> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/padded.cc
> >> @@ -0,0 +1,537 @@
> >> +// { dg-do run { target c++26 } }
> >> +#include <mdspan>
> >> +
> >> +#include <cstdint>
> >> +#include "../int_like.h"
> >> +#include <testsuite_hooks.h>
> >> +
> >> +constexpr size_t dyn = std::dynamic_extent;
> >> +
> >> +constexpr bool
> >> +test_default_ctor()
> >> +{
> >> + using Extents = std::extents<size_t, 3, 5>;
> >> +
> >> + std::layout_left_padded<2>::mapping<Extents> msta;
> >> + VERIFY(msta.stride(0) == 1);
> >> + VERIFY(msta.stride(1) == 4);
> >> +
> >> + std::layout_left_padded<dyn>::mapping<Extents> mdyn;
> >> + VERIFY(mdyn.stride(0) == 1);
> >> + VERIFY(mdyn.stride(1) == 3);
> >> + return true;
> >> +}
> >> +
> >> +constexpr bool
> >> +test_from_exts()
> >> +{
> >> + auto exts = std::dextents<size_t, 2>{3, 5};
> >> +
> >> + std::layout_left_padded<0>::mapping m0_sta(exts);
> >> + VERIFY(m0_sta.stride(1) == exts.extent(0));
> >> +
> >> + std::layout_left_padded<1>::mapping m1_sta(exts);
> >> + VERIFY(m1_sta.stride(1) == exts.extent(0));
> >> +
> >> + std::layout_left_padded<2>::mapping m2_sta(exts);
> >> + VERIFY(m2_sta.stride(1) == 4);
> >> +
> >> + std::layout_left_padded<dyn>::mapping mdyn(exts);
> >> + VERIFY(mdyn.stride(1) == exts.extent(0));
> >> + return true;
> >> +}
> >> +
> >> +template<size_t PaddingValue, typename CustomPadType>
> >> +constexpr bool
> >> +test_from_pad()
> >> +{
> >> + using Layout = std::layout_left_padded<PaddingValue>;
> >> + auto pad = 3;
> >> + auto exts = std::dextents<size_t, 3>{pad + 1, 5, 7};
> >> + typename Layout::mapping m(exts, CustomPadType{pad});
> >> + VERIFY(std::cmp_equal(m.stride(1), 2*pad));
> >> + VERIFY(m.extents() == exts);
> >> + return true;
> >> +}
> >> +
> >> +template<size_t PaddingValue>
> >> +constexpr void
> >> +test_from_pad_all()
> >> +{
> >> + test_from_pad<PaddingValue, int>();
> >> + static_assert(test_from_pad<PaddingValue, int>());
> >> +
> >> + test_from_pad<PaddingValue, IntLike>();
> >> + test_from_pad<PaddingValue, MutatingInt>();
> >> + test_from_pad<PaddingValue, RValueInt>();
> >> +
> >> + using Layout = std::layout_left_padded<PaddingValue>;
> >> + using Extents = std::dims<3>;
> >> + using Mapping = Layout::template mapping<Extents>;
> >> + static_assert(!std::is_constructible_v<Mapping, Extents,
> ThrowingInt>);
> >> + static_assert(!std::is_constructible_v<Mapping, Extents,
> NotIntLike>);
> >> +}
> >> +
> >> +constexpr bool
> >> +is_same_mapping(const auto& lhs, const auto& rhs)
> >> +{
> >> + if (lhs.extents().rank() != rhs.extents().rank())
> >> + return false;
> >> +
> >> + if (lhs.extents() != rhs.extents())
> >> + return false;
> >> +
> >> + for (size_t i = 0; i < lhs.extents().rank(); ++i)
> >> + if (lhs.stride(i) != rhs.stride(i))
> >> + return false;
> >> + return true;
> >> +}
> >> +
> >> +enum class ConversionRule
> >> +{
> >> + Never,
> >> + Always,
> >> + Conventional
> >> +};
> >> +
> >> +template<typename To, typename From>
> >> +consteval bool
> >> +should_convert(auto rule)
> >> +{
> >> + if constexpr (rule == ConversionRule::Never)
> >> + return false;
> >> + if constexpr (rule == ConversionRule::Always)
> >> + return true;
> >> + else
> >> + return std::is_convertible_v<typename From::extents_type,
> >> + typename To::extents_type>;
> >> +}
> >> +
> >> +template<typename To, typename From>
> >> + constexpr void
> >> + check_convertible(const From& m, auto conversion_rule)
> >> + {
> >> + VERIFY(is_same_mapping(m, To(m)));
> >> + constexpr bool expected = should_convert<To,
> From>(conversion_rule);
> >> + static_assert(std::is_convertible_v<From, To> == expected);
> >> + }
> >> +
> >> +template<typename LayoutTo, typename Esta, typename Edyn, typename
> Ewrong>
> >> +constexpr void
> >> +check_convertible_variants(auto msta, auto conversion_rule)
> >> +{
> >> + using LayoutFrom = decltype(msta)::layout_type;
> >> + constexpr auto cregular = std::cw<ConversionRule::Conventional>;
> >> +
> >> + // There's a twist when both mappings are left-padded. There's two
> >> distinct
> >> + // ctors: a defaulted copy ctor and a constrained template that
> enables
> >> + // construction from left-padded mappings even if their layout_type
> is
> >> + // different. The two ctors have different rules regarding
> conversion.
> >> +
> >> + if constexpr (!std::same_as<LayoutTo, LayoutFrom>)
> >> + check_convertible<typename LayoutTo::mapping<Esta>>(msta,
> >> conversion_rule);
> >> + else
> >> + check_convertible<typename LayoutTo::mapping<Esta>>(msta,
> cregular);
> >> +
> >> + check_convertible<typename LayoutTo::mapping<Edyn>>(msta,
> >> conversion_rule);
> >> +
> >> + auto mdyn = typename LayoutFrom::mapping<Edyn>(msta);
> >> + check_convertible<typename LayoutTo::mapping<Esta>>(mdyn,
> >> conversion_rule);
> >> + if constexpr (!std::same_as<LayoutTo, LayoutFrom>)
> >> + check_convertible<typename LayoutTo::mapping<Edyn>>(mdyn,
> >> conversion_rule);
> >> + else
> >> + check_convertible<typename LayoutTo::mapping<Edyn>>(mdyn,
> cregular);
> >> +
> >> + static_assert(!std::is_constructible_v<
> >> + typename LayoutTo::mapping<Esta>, typename
> >> LayoutFrom::mapping<Ewrong>>);
> >> +};
> >> +
> >> +template<typename PaddedLayout>
> >> + constexpr void
> >> + test_from_left_1d()
> >> + {
> >> + using E1 = std::extents<int, 6>;
> >> + using E2 = std::extents<int, dyn>;
> >> + using E3 = std::extents<int, 5>;
> >> + constexpr auto cr = std::cw<ConversionRule::Conventional>;
> >> +
> >> + auto msta = std::layout_left::mapping(E1{});
> >> + check_convertible_variants<PaddedLayout, E1, E2, E3>(msta, cr);
> >> + }
> >> +
> >> +template<typename PaddedLayout>
> >> + constexpr void
> >> + test_from_left_2d()
> >> + {
> >> + using E1 = std::extents<int, 6, 5>;
> >> + using E2 = std::extents<int, dyn, 5>;
> >> + using E3 = std::extents<int, 6, 6>;
> >> + constexpr auto cr = std::cw<ConversionRule::Conventional>;
> >> +
> >> + auto msta = std::layout_left::mapping(E1{});
> >> + check_convertible_variants<PaddedLayout, E1, E2, E3>(msta, cr);
> >> + }
> >> +
> >> +constexpr bool
> >> +test_from_left()
> >> +{
> >> + auto check = []<typename PaddedLayout>(PaddedLayout)
> >> + {
> >> + test_from_left_1d<PaddedLayout>();
> >> + test_from_left_2d<PaddedLayout>();
> >> + };
> >> +
> >> + check(std::layout_left_padded<0>{});
> >> + check(std::layout_left_padded<1>{});
> >> + check(std::layout_left_padded<2>{});
> >> + check(std::layout_left_padded<6>{});
> >> + check(std::layout_left_padded<dyn>{});
> >> +
> >> + // rank == 1 is more permissive:
> >> + test_from_left_1d<std::layout_left_padded<5>>();
> >> + return true;
> >> +}
> >> +
> >> +constexpr bool
> >> +test_from_stride()
> >> +{
> >> + using E1 = std::extents<size_t, 6, 5, 7>;
> >> + using E2 = std::dims<3>;
> >> + using E3 = std::extents<size_t, 6, 6, 7>;
> >> +
> >> + auto check = []<typename PaddedLayout>(PaddedLayout)
> >> + {
> >> + auto exts = E1{};
> >> + auto strides = std::array<int, 3>{
> >> + 1, exts.extent(0), exts.extent(0) * exts.extent(1)};
> >> + constexpr auto cr = std::cw<ConversionRule::Never>;
> >> +
> >> + auto m = std::layout_stride::mapping(exts, strides);
> >> + check_convertible_variants<PaddedLayout, E1, E2, E3>(m, cr);
> >> + };
> >> +
> >> + check(std::layout_left_padded<0>{});
> >> + check(std::layout_left_padded<1>{});
> >> + check(std::layout_left_padded<2>{});
> >> + check(std::layout_left_padded<6>{});
> >> + check(std::layout_left_padded<dyn>{});
> >> + return true;
> >> +}
> >> +
> >> +constexpr bool
> >> +test_from_leftpad_0d()
> >> +{
> >> + using E1 = std::extents<uint16_t>;
> >> + using E2 = std::extents<uint8_t>;
> >> + using E3 = std::extents<uint8_t, 1>;
> >> +
> >> + std::layout_left_padded<6>::mapping<E1> msta{E1{}};
> >> +
> >> + auto check = []<typename To>(To, auto m)
> >> + {
> >> + constexpr auto cr = std::cw<ConversionRule::Always>;
> >> + check_convertible_variants<To, E1, E2, E3>(m, cr);
> >> + };
> >> +
> >> + check(std::layout_left_padded<6>{}, msta);
> >> + check(std::layout_left_padded<dyn>{}, msta);
> >> + return true;
> >> +}
> >> +
> >> +constexpr bool
> >> +test_from_leftpad_1d()
> >> +{
> >> + using E1 = std::extents<int, 6>;
> >> + using E2 = std::extents<int, dyn>;
> >> + using E3 = std::extents<int, 6, 6>;
> >> +
> >> + std::layout_left_padded<6>::mapping<E1> msta{E1{}};
> >> + std::layout_left_padded<dyn>::mapping<E1> mdyn{E1{}};
> >> +
> >> + auto check = []<typename To>(To, auto m)
> >> + {
> >> + constexpr auto cr = std::cw<ConversionRule::Always>;
> >> + check_convertible_variants<To, E1, E2, E3>(m, cr);
> >> + };
> >> +
> >> + // Remember, for rank <= 1 the padding_value is irrelevant.
> >> + check(std::layout_left_padded<6>{}, msta);
> >> + check(std::layout_left_padded<6>{}, mdyn);
> >> + check(std::layout_left_padded<dyn>{}, msta);
> >> + check(std::layout_left_padded<dyn>{}, mdyn);
> >> + return true;
> >> +}
> >> +
> >> +constexpr bool
> >> +test_from_leftpad_2d()
> >> +{
> >> + using E1 = std::extents<int, 6, 5>;
> >> + using E2 = std::extents<int, dyn, 5>;
> >> + using E3 = std::extents<int, 6, 6>;
> >> +
> >> + std::layout_left_padded<6>::mapping<E1> msta{E1{}};
> >> + std::layout_left_padded<dyn>::mapping<E1> mdyn{E1{}};
> >> +
> >> + constexpr auto calways = std::cw<ConversionRule::Always>;
> >> + constexpr auto cnever = std::cw<ConversionRule::Never>;
> >> +
> >> + auto check = []<typename To>(To, auto m, auto cr)
> >> + { check_convertible_variants<To, E1, E2, E3>(m, cr); };
> >> +
> >> + check(std::layout_left_padded<6>{}, msta, cnever);
> >> + check(std::layout_left_padded<6>{}, mdyn, cnever);
> >> + check(std::layout_left_padded<dyn>{}, msta, calways);
> >> + check(std::layout_left_padded<dyn>{}, mdyn, cnever);
> >> + return true;
> >> +}
> >> +
> >> +constexpr bool
> >> +test_from_right()
> >> +{
> >> + using E1 = std::extents<size_t, 3>;
> >> + using E2 = std::dims<1>;
> >> + using E3 = std::extents<size_t, 5>;
> >> + constexpr auto cr = std::cw<ConversionRule::Conventional>;
> >> +
> >> + auto msta = std::layout_right::mapping(E1{});
> >> +
> >> + // Remember, the padding_value has no effect for rank <= 1.
> >> + check_convertible_variants<std::layout_left_padded<0>, E1, E2,
> >> E3>(msta, cr);
> >> + check_convertible_variants<std::layout_left_padded<1>, E1, E2,
> >> E3>(msta, cr);
> >> + check_convertible_variants<std::layout_left_padded<2>, E1, E2,
> >> E3>(msta, cr);
> >> + check_convertible_variants<std::layout_left_padded<5>, E1, E2,
> >> E3>(msta, cr);
> >> + check_convertible_variants<std::layout_left_padded<6>, E1, E2,
> >> E3>(msta, cr);
> >> + check_convertible_variants<std::layout_left_padded<dyn>, E1, E2,
> >> E3>(msta, cr);
> >> + return true;
> >> +}
> >> +
> >> +constexpr bool
> >> +test_to_left()
> >> +{
> >> + using E1 = std::extents<int, 6, 5>;
> >> + using E2 = std::extents<int, dyn, 5>;
> >> + using E3 = std::extents<int, 6, 6>;
> >> +
> >> + auto exts_sta = E1{};
> >> +
> >> + auto check = [](auto msta)
> >> + {
> >> + constexpr auto cr = std::cw<ConversionRule::Conventional>;
> >> + check_convertible_variants<std::layout_left, E1, E2, E3>(msta, cr);
> >> + };
> >> +
> >> + check(std::layout_left_padded<0>::mapping(exts_sta));
> >> + check(std::layout_left_padded<2>::mapping(exts_sta));
> >> + check(std::layout_left_padded<6>::mapping(exts_sta));
> >> + check(std::layout_left_padded<dyn>::mapping(exts_sta, 0));
> >> + check(std::layout_left_padded<dyn>::mapping(exts_sta, 2));
> >> + check(std::layout_left_padded<dyn>::mapping(exts_sta, 6));
> >> + return true;
> >> +}
> >> +
> >> +constexpr bool
> >> +test_never_to_right()
> >> +{
> >> + using E1 = std::extents<size_t, 3>;
> >> + using E2 = std::dims<1>;
> >> +
> >> + auto mr_sta = std::layout_right::mapping(E1{});
> >> + auto mlp_sta = std::layout_left_padded<2>::mapping<E1>{mr_sta};
> >> + static_assert(!std::is_constructible_v<decltype(mr_sta),
> >> decltype(mlp_sta)>);
> >> +
> >> + auto mr_dyn = std::layout_right::mapping(E2{E1{}});
> >> + auto mlp_dyn = std::layout_left_padded<2>::mapping<E2>{mr_dyn};
> >> + static_assert(!std::is_constructible_v<decltype(mr_dyn),
> >> decltype(mlp_dyn)>);
> >> + return true;
> >> +}
> >> +
> >> +template<typename Layout>
> >> +constexpr void
> >> +test_strides()
> >> +{
> >> + auto check = [](auto exts)
> >> + {
> >> + auto m = typename Layout::mapping(exts);
> >> + using IndexType = typename decltype(m)::index_type;
> >> + constexpr size_t rank = decltype(m)::extents_type::rank();
> >> +
> >> + auto strides = m.strides();
> >> + static_assert(std::same_as<decltype(strides), std::array<IndexType,
> >> rank>>);
> >> + VERIFY(strides.size() == rank);
> >> + for (size_t i = 0; i < strides.size(); ++i)
> >> + VERIFY(strides[i] == m.stride(i));
> >> + };
> >> +
> >> + check(std::extents());
> >> + check(std::extents(0));
> >> + check(std::extents(3));
> >> + check(std::extents(3, 5, 7));
> >> + check(std::extents(3, 0, 7));
> >> +}
> >> +
> >> +constexpr bool
> >> +test_strides_all()
> >> +{
> >> + test_strides<std::layout_left_padded<0>>();
> >> + test_strides<std::layout_left_padded<1>>();
> >> + test_strides<std::layout_left_padded<3>>();
> >> + test_strides<std::layout_left_padded<dyn>>();
> >> + return true;
> >> +}
> >> +
> >> +constexpr void
> >> +test_exhaustive_0d()
> >> +{
> >> + auto exts = std::extents<int>{};
> >> +
> >> + auto check = [](auto m)
> >> + {
> >> + static_assert(m.is_always_exhaustive());
> >> + VERIFY(m.is_exhaustive());
> >> + };
> >> +
> >> + check(std::layout_left_padded<0>::mapping(exts));
> >> + check(std::layout_left_padded<1>::mapping(exts));
> >> + check(std::layout_left_padded<2>::mapping(exts));
> >> + check(std::layout_left_padded<dyn>::mapping(exts));
> >> +}
> >> +
> >> +constexpr void
> >> +test_exhaustive_1d()
> >> +{
> >> + auto check_dyn_and_sta = []<typename Layout>(Layout)
> >> + {
> >> + auto check = [](auto exts)
> >> + {
> >> + auto m = typename Layout::mapping(exts);
> >> + static_assert(m.is_always_exhaustive());
> >> + VERIFY(m.is_exhaustive());
> >> + };
> >> +
> >> + check(std::extents(4));
> >> + check(std::extents<int, 4>{});
> >> + };
> >> +
> >> + check_dyn_and_sta(std::layout_left_padded<1>{});
> >> + check_dyn_and_sta(std::layout_left_padded<2>{});
> >> + check_dyn_and_sta(std::layout_left_padded<6>{});
> >> + check_dyn_and_sta(std::layout_left_padded<dyn>{});
> >> +}
> >> +
> >> +constexpr void
> >> +test_exhaustive_3d()
> >> +{
> >> + auto exts_dyn = std::extents(4, 5, 7);
> >> + auto exts_sta = std::extents<int, 4, 5, 7>{};
> >> + auto ctrue = std::cw<true>;
> >> + auto cfalse= std::cw<false>;
> >> +
> >> + auto check = [](auto m, auto static_expected, auto runtime_expected)
> >> + {
> >> + static_assert(m.is_always_exhaustive() == static_expected);
> >> + VERIFY(m.is_exhaustive() == runtime_expected);
> >> + };
> >> +
> >> + check(std::layout_left_padded<0>::mapping(exts_sta), ctrue, true);
> >> + check(std::layout_left_padded<0>::mapping(exts_dyn), cfalse, true);
> >> + check(std::layout_left_padded<1>::mapping(exts_sta), ctrue, true);
> >> + check(std::layout_left_padded<1>::mapping(exts_dyn), cfalse, true);
> >> + check(std::layout_left_padded<2>::mapping(exts_sta), ctrue, true);
> >> + check(std::layout_left_padded<2>::mapping(exts_dyn), cfalse, true);
> >> + check(std::layout_left_padded<6>::mapping(exts_dyn), cfalse, false);
> >> + check(std::layout_left_padded<6>::mapping(exts_sta), cfalse, false);
> >> + check(std::layout_left_padded<dyn>::mapping(exts_sta), cfalse, true);
> >> + check(std::layout_left_padded<dyn>::mapping(exts_dyn, 2), cfalse,
> true);
> >> + check(std::layout_left_padded<dyn>::mapping(exts_dyn, 3), cfalse,
> >> false);
> >> +}
> >> +
> >> +constexpr bool
> >> +test_exhaustive()
> >> +{
> >> + test_exhaustive_0d();
> >> + test_exhaustive_1d();
> >> + test_exhaustive_3d();
> >> + return true;
> >> +}
> >> +
> >> +constexpr bool
> >> +test_op_eq()
> >> +{
> >> + // The generic cases are handled in layouts/mapping.cc. Here we check
> >> special
> >> + // cases related to non exhaustive layouts.
> >> + using E1 = std::extents<size_t, 6, 5, 7>;
> >> + using E2 = std::dims<3>;
> >> + using E3 = std::extents<size_t, 7, 5, 7>;
> >> +
> >> + auto m1 = std::layout_left_padded<0>::mapping(E1{});
> >> + auto m2 = std::layout_left_padded<7>::mapping(E1{});
> >> +
> >> + VERIFY(m1 == std::layout_left_padded<0>::mapping(E1{}));
> >> + VERIFY(m1 == std::layout_left_padded<1>::mapping(E1{}));
> >> + VERIFY(m1 == std::layout_left_padded<2>::mapping(E1{}));
> >> + VERIFY(m1 == std::layout_left_padded<6>::mapping(E1{}));
> >> + VERIFY(m1 != std::layout_left_padded<7>::mapping(E1{}));
> >> + VERIFY(m1 == std::layout_left_padded<dyn>::mapping(E1{}));
> >> + VERIFY(m1 != std::layout_left_padded<dyn>::mapping(E1{}, 7));
> >> +
> >> + VERIFY(m1 == std::layout_left_padded<0>::mapping(E2{E1{}}));
> >> + VERIFY(m1 == std::layout_left_padded<1>::mapping(E2{E1{}}));
> >> + VERIFY(m1 == std::layout_left_padded<2>::mapping(E2{E1{}}));
> >> + VERIFY(m1 == std::layout_left_padded<6>::mapping(E2{E1{}}));
> >> + VERIFY(m1 != std::layout_left_padded<7>::mapping(E2{E1{}}));
> >> + VERIFY(m1 == std::layout_left_padded<dyn>::mapping(E2{E1{}}));
> >> + VERIFY(m1 != std::layout_left_padded<dyn>::mapping(E2{E1{}}, 7));
> >> +
> >> + VERIFY(m2 == std::layout_left_padded<7>::mapping(E1{}));
> >> + VERIFY(m2 == std::layout_left_padded<dyn>::mapping(E1{}, 7));
> >> + VERIFY(m2 == std::layout_left_padded<7>::mapping(E2{E1{}}));
> >> + VERIFY(m2 == std::layout_left_padded<dyn>::mapping(E2{E1{}}, 7));
> >> +
> >> + VERIFY(m2 != std::layout_left_padded<7>::mapping(E3{}));
> >> + VERIFY(m2 != std::layout_left_padded<dyn>::mapping(E3{}, 7));
> >> + return true;
> >> +}
> >> +
> >> +int
> >> +main()
> >> +{
> >> + test_default_ctor();
> >> + static_assert(test_default_ctor());
> >> +
> >> + test_from_exts();
> >> + static_assert(test_from_exts());
> >> +
> >> + test_from_left();
> >> + static_assert(test_from_left());
> >> +
> >> + test_from_pad_all<dyn>();
> >> + test_from_pad_all<3>();
> >> +
> >> + test_from_stride();
> >> + static_assert(test_from_stride());
> >> +
> >> + test_from_leftpad_0d();
> >> + static_assert(test_from_leftpad_0d());
> >> +
> >> + test_from_leftpad_1d();
> >> + static_assert(test_from_leftpad_1d());
> >> +
> >> + test_from_leftpad_2d();
> >> + static_assert(test_from_leftpad_2d());
> >> +
> >> + test_from_right();
> >> + static_assert(test_from_right());
> >> +
> >> + test_to_left();
> >> + static_assert(test_to_left());
> >> +
> >> + test_never_to_right();
> >> + static_assert(test_never_to_right());
> >> +
> >> + test_strides_all();
> >> + static_assert(test_strides_all());
> >> +
> >> + test_exhaustive();
> >> + static_assert(test_exhaustive());
> >> + return 0;
> >> +}
> >> diff --git
> >> a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/padded_neg.cc
> >> b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/padded_neg.cc
> >> new file mode 100644
> >> index 00000000000..83b4402d8c5
> >> --- /dev/null
> >> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/padded_neg.cc
> >> @@ -0,0 +1,225 @@
> >> +// { dg-do compile { target c++26 } }
> >> +#include <mdspan>
> >> +
> >> +#include <cstdint>
> >> +
> >> +constexpr size_t dyn = std::dynamic_extent;
> >> +
> >> +constexpr bool
> >> +test_from_extens_oversized()
> >> +{
> >> + using E1 = std::dextents<uint16_t, 2>;
> >> + using E2 = std::dextents<uint8_t, 2>;
> >> +
> >> + auto mlp = std::layout_left_padded<dyn>::mapping(E1{8, 128});
> >> + auto m = std::layout_left_padded<dyn>::mapping<E2>(mlp);
> >> + (void) m;
> >> + return true;
> >> +}
> >> +
> >> +constexpr bool
> >> +test_from_left()
> >> +{
> >> + auto exts = std::extents<uint8_t, 6, dyn>{4};
> >> + auto ml = std::layout_left::mapping(exts);
> >> +
> >> + using Layout = std::layout_left_padded<4>;
> >> + Layout::mapping<decltype(exts)> m(ml); // { dg-error "from here" }
> >> + return true;
> >> +}
> >> +static_assert(test_from_left());
> >> +
> >> +constexpr bool
> >> +test_from_left_bad_runtime_stride()
> >> +{
> >> + auto exts = std::extents<uint8_t, dyn, dyn>{6, 4};
> >> + auto ml = std::layout_left::mapping(exts);
> >> +
> >> + using Layout = std::layout_left_padded<4>;
> >> + Layout::mapping<decltype(exts)> m(ml); // { dg-error "expansion of" }
> >> + return true;
> >> +}
> >> +static_assert(test_from_left_bad_runtime_stride());
> >> +
> >> +constexpr bool
> >> +test_from_left_oversized_extents()
> >> +{
> >> + auto exts = std::extents<uint16_t, dyn, dyn>{8, 128};
> >> + auto ml = std::layout_left::mapping(exts);
> >> +
> >> + using Layout = std::layout_left_padded<8>;
> >> + Layout::mapping<std::extents<uint8_t, dyn, dyn>> m(ml); // { dg-error
> >> "expansion of" }
> >> + return true;
> >> +}
> >> +static_assert(test_from_left_oversized_extents());
> >> +
> >> +template<size_t PaddingValue>
> >> + constexpr bool
> >> + test_pad_overflow()
> >> + {
> >> + auto exts = std::extents<uint8_t, dyn>{4};
> >> + auto n = size_t(1) << 9;
> >> + auto m = typename
> >> std::layout_left_padded<PaddingValue>::mapping(exts, n);
> >> + (void) m;
> >> + return true;
> >> + }
> >> +static_assert(test_pad_overflow<1>()); // { dg-error "expansion of" }
> >> +static_assert(test_pad_overflow<dyn>()); // { dg-error "expansion of" }
> >> +
> >> +template<size_t PaddingValue>
> >> + constexpr bool
> >> + test_from_pad_negative()
> >> + {
> >> + auto exts = std::extents(4);
> >> + auto m = typename
> >> std::layout_left_padded<PaddingValue>::mapping(exts, -1);
> >> + (void) m;
> >> + return true;
> >> + }
> >> +static_assert(test_from_pad_negative<1>()); // { dg-error "expansion
> >> of" }
> >> +static_assert(test_from_pad_negative<dyn>()); // { dg-error "expansion
> >> of" }
> >> +
> >> +template<size_t Pad>
> >> + constexpr bool
> >> + test_static_pad_same()
> >> + {
> >> + using Extents = std::extents<int, dyn>;
> >> + using Mapping = typename
> >> std::layout_left_padded<Pad>::mapping<Extents>;
> >> + auto exts = Extents{4};
> >> + auto m = Mapping(exts, Pad + 1); // { dg-error "expansion of"
> }
> >> + (void) m;
> >> + return true;
> >> + }
> >> +static_assert(test_static_pad_same<1>()); // { dg-error "expansion of"
> }
> >> +static_assert(test_static_pad_same<3>()); // { dg-error "expansion of"
> }
> >> +
> >> +constexpr bool
> >> +test_from_stride_wrong_stride0()
> >> +{
> >> + using Extents = std::dextents<size_t, 2>;
> >> + auto e = Extents{3, 5};
> >> + auto ms = std::layout_stride::mapping(e, std::array<int, 2>{2, 7});
> >> + auto m = std::layout_left_padded<dyn>::mapping<Extents>(ms); // {
> >> dg-error "expansion of" }
> >> + (void) m;
> >> + return true;
> >> +}
> >> +static_assert(test_from_stride_wrong_stride0());
> >> +
> >> +constexpr bool
> >> +test_from_stride_wrong_stride1()
> >> +{
> >> + using Extents = std::dextents<size_t, 2>;
> >> + auto e = Extents{3, 5};
> >> + auto ms = std::layout_stride::mapping(e, std::array<int, 2>{1, 3});
> >> + auto m = std::layout_left_padded<2>::mapping<Extents>(ms); // {
> >> dg-error "expansion of" }
> >> + (void) m;
> >> + return true;
> >> +}
> >> +static_assert(test_from_stride_wrong_stride1());
> >> +
> >> +constexpr bool
> >> +test_from_stride_wrong_stride2()
> >> +{
> >> + using Extents = std::dims<3>;
> >> + auto e = Extents{3, 5, 7};
> >> + auto ms = std::layout_stride::mapping(e, std::array<int, 3>{1, 4,
> 21});
> >> + auto m = std::layout_left_padded<dyn>::mapping<Extents>(ms); // here
> >> (not implemented)
> >> + (void) m;
> >> + return true;
> >> +}
> >> +static_assert(test_from_stride_wrong_stride2());
> >> +
> >> +constexpr bool
> >> +test_from_stride_oversized()
> >> +{
> >> + auto exts = std::extents<uint16_t, dyn, dyn>{2, 6};
> >> + auto ms = std::layout_stride::mapping(exts, std::array<uint16_t,
> 2>{1,
> >> 128});
> >> +
> >> + using Layout = std::layout_left_padded<dyn>;
> >> + Layout::mapping<std::extents<uint8_t, dyn, dyn>> m(ms); // { dg-error
> >> "expansion of" }
> >> + (void) m;
> >> + return true;
> >> +}
> >> +static_assert(test_from_stride_oversized());
> >> +
> >> +constexpr bool
> >> +test_from_leftpad_dyn()
> >> +{
> >> + using Extents = std::dextents<size_t, 2>;
> >> + auto e = Extents{3, 5};
> >> + auto mlp = std::layout_left_padded<dyn>::mapping(e);
> >> + auto m = std::layout_left_padded<2>::mapping<Extents>(mlp); // {
> >> dg-error "expansion of" }
> >> + (void) m;
> >> + return true;
> >> +}
> >> +static_assert(test_from_leftpad_dyn());
> >> +
> >> +constexpr bool
> >> +test_from_leftpad_sta()
> >> +{
> >> + using Extents = std::dextents<size_t, 2>;
> >> + auto e = Extents{3, 5};
> >> + auto mlp = std::layout_left_padded<3>::mapping(e);
> >> + auto m = std::layout_left_padded<2>::mapping<Extents>(mlp); // {
> >> dg-error "required from" }
> >> + (void) m;
> >> + return true;
> >> +}
> >> +static_assert(test_from_leftpad_sta());
> >> +
> >> +constexpr bool
> >> +test_from_leftpad_oversized()
> >> +{
> >> + using E1 = std::extents<uint16_t, 8, 128>;
> >> + using E2 = std::extents<uint8_t, dyn, dyn>;
> >> + auto mlp = std::layout_left_padded<dyn>::mapping<E1>(E1{});
> >> + auto m = std::layout_left_padded<dyn>::mapping<E2>(mlp); // {
> dg-error
> >> "expansion of" }
> >> + (void) m;
> >> + return true;
> >> +}
> >> +static_assert(test_from_leftpad_oversized());
> >> +
> >> +template<size_t RunId>
> >> + constexpr bool
> >> + test_to_left_not_exhaustive()
> >> + {
> >> + using E1 = std::extents<int, 6, 5>;
> >> + using E2 = std::extents<int, dyn, 5>;
> >> +
> >> + [[maybe_unused]] auto msta =
> >> std::layout_left_padded<7>::mapping(E1{});
> >> + if constexpr (RunId == 0)
> >> + {
> >> + auto m = std::layout_left::mapping<E1>(msta); // { dg-error
> >> "required from" }
> >> + (void) m;
> >> + }
> >> + if constexpr (RunId == 1)
> >> + {
> >> + auto m = std::layout_left::mapping<E2>(msta); // { dg-error
> >> "expansion of" }
> >> + (void) m;
> >> + }
> >> +
> >> + [[maybe_unused]] auto mdyn =
> >> std::layout_left_padded<dyn>::mapping(E2{E1{}}, 7);
> >> + if constexpr (RunId == 2)
> >> + {
> >> + auto m = std::layout_left::mapping<E1>(mdyn); // { dg-error
> >> "expansion of" }
> >> + (void) m;
> >> + }
> >> + if constexpr (RunId == 3)
> >> + {
> >> + auto m = std::layout_left::mapping<E2>(mdyn); // { dg-error
> >> "expansion of" }
> >> + (void) m;
> >> + }
> >> + return true;
> >> + }
> >> +static_assert(test_to_left_not_exhaustive<0>());
> >> +static_assert(test_to_left_not_exhaustive<1>());
> >> +static_assert(test_to_left_not_exhaustive<2>());
> >> +static_assert(test_to_left_not_exhaustive<3>());
> >> +
> >> +// { dg-prune-output "padding_value must be representable as
> index_type" }
> >> +// { dg-prune-output "non-constant condition for static assertion" }
> >> +// { dg-prune-output "called in a constant expression" }
> >> +// { dg-prune-output "no matching function" }
> >> +// { dg-prune-output "static assertion failed" }
> >> +// { dg-prune-output "__glibcxx_assert_fail()" }
> >> +// { dg-prune-output "must be compatible with other.stride" }
> >> +// { dg-prune-output "padding_value is dynamic_extent" }
> >> +// { dg-prune-output "_S_rank <= 1" }
> >> --
> >> 2.50.1
> >>
> >>
> >
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://gcc.gnu.org/pipermail/libstdc++/attachments/20250911/14cf47c1/attachment-0001.htm>
More information about the Libstdc++
mailing list